Quarl-antiquark bound states

This paper discusses various aspects of the description of hadrons as bound states of quarks in the simpler example of mesons, which are regarded as being composite of a (constituent) quark and an antiquark. it is shown how a recently derived relativistic virial theorem may be used to cast some light on the interplay between the (semi)relativistic and (maybe only effectively) nonrelativistic treatment of bound states. Furthermore, a new relativistic approach to fermion-antifermion bound states is sketched and applied to predict analytically a few selected general features of the meson spectrum